The electronics, display and energy industries rely on the formation of coatings and patterns of conductive and other electronically active materials to form circuits on organic and inorganic substrates. The primary methods for generating these patterns are screen printing for features larger than about 100 μm and thin film and etching methods for features smaller than about 100 μm. Other subtractive methods to attain fine feature sizes include the use of photo-patternable pastes and laser trimming.
It is the trend in the electronics industry to make smaller and less expensive electronic devices that provide higher resolution and enhanced display performance. As a result, it has become necessary to develop new materials and new approaches to manufacture such devices.
Photo-patterning technologies offer uniform finer lines and space resolution when compared to traditional screen-printing methods. A photo-patterning method, such as DuPont's FODEL@ printing system, utilizes a photoimageable organic medium as found in patents U.S. Pat. Nos. 4,912,019; 4,925,771; and 5,049,480, whereby the substrate is first completely covered (printed, sprayed, coated or laminated) with the photoimageable thick film composition and dried. An image of the circuit pattern is generated by exposure of the photoimageable thick film composition with actinic radiation through a photomask bearing a circuit pattern. Actinic radiation is radiation such as ultraviolet that can cause photochemical reactions. The exposed substrate is then developed. The unexposed portion of the circuit pattern is washed away leaving the photoimaged thick film composition on the substrate that, subsequently, is fired to remove all remaining organic materials and sinter inorganic materials. Such a photo-patterning method demonstrates line resolution of about 30 microns depending on the substrate smoothness, inorganic particle size distribution, exposure, and development variables. When employed for the production of conductors in display devices such as plasma display panels, field emission displays, or liquid crystal displays, the conducting lines can be up to a meter long, many orders of magnitude longer than their widths and precision. The process is necessarily subtractive in its nature as a result of the wash-out of a large portion of the pattern. A process that is additive is desired by those in the industry.
The ink jet printing system is a high resolution, additive printing system having the ability to print complex patterns through digital instructions. In the field of conductive inks, a liquid dispersion of ultrafine metal particles has been used in the formation of a conductive circuit making use of the ink jet printing system (US patent application 2003/0110978 A1). Liquid dispersions of other ultrafine particles such as metal oxides, organometallics or polymers can also be used in the formation of components of electronic circuits or display devices using ink jet printing systems. Ink jet techniques necessarily require low viscosity fluids for proper operation of the jetting system. It is difficult to build features to any appreciable thickness, though this can be done utilizing multiple passes. Drying time or some other means for stabilizing the initial feature is required between passes. Resolution is often compromised and it is difficult to obtain appreciable feature height to feature width because non-viscous, wetting fluids are employed. These systems are prone to clogging when employing inks with high solids content, particularly when the solid particles are not spherical.
United States patent application US 2004 (009290) discloses the spinning of conductive fibers or ribbons that are attached to a substrate by spinning a fiber or ribbon composed of an organic polymer with an inorganic material and affixing that fiber or ribbon in a desired orientation on a substrate and finally heating the composition to remove the organic polymer. This results in the conductive fiber or ribbon being affixed to the substrate in the desired orientation. Suitable inorganic materials are generally metal conductors that include Au, Ni, Au—Cr alloy, Au—Ta alloy, Cu—Cr alloy, Au/Indium tin oxide, Cu, Ag, and Ni. These constructions are useful as electrodes particularly on silicon wafers in solar cell fabrication. While several spinning methods are discussed, the concept of utilizing a viscoelastic system in which the concentration of the polymer component is no higher than a few percent is not disclosed. United States patent application US 2004050476 is similar but directed to a process for the fabrication of features on a display panel utilizing fibers or ribbons comprising organic polymers and inorganic material, the inorganic materials being phosphors, conductive metals or dielectric particles. These applications do not anticipate the advantage of using a viscoelastic polymer solution having a low polymer content thereby maximizing the quantity of functional phase materials printed onto the substrate.
It is possible to disperse solids in many synthetic polymers and spin fibers of those polymers. This is practiced with carbon blacks (U.S. Pat. Nos. 4,129,677, 4,388,370, and EP 250664), zinc oxide (U.S. Pat. No. 5,391,432), magnesium oxide (JP 57161115), or antimony tin oxide-coated Ti oxide particles (JP 59047474) in nylon for antistatic purpose, both throughout the fiber and in segregated into the core of core-shell compositions. Tin oxide (JP 49034550) has been added as a flame retardant. In all of these examples, the polymer component is an appreciable fraction and usually the majority of the mixture being spun. Conductivities of the resulting system are relatively low because the content of the active phase is necessarily so low. In addition, the fibers would have to be adhered to the substrate surface at very high temperatures for them to adhere. To achieve higher conductivities, the polymer fraction would have to be fired out, but the polymer content is so high that the volatilization process would destroy the lines.
An advantage of the composition disclosed herein is that the polymer content can be lower than the content of the functional phase and that it can be spun and adhered to the substrate surface at conditions close to ambient.
Despite the foregoing advances in such systems, manufacturers are continuously seeking compositions with improved utility of the ultrafine materials and finer resolution of lines and spaces. Such materials will increase the speed of the manufacturing processes without compromising high resolutions in the lines and spaces of the circuit or display patterns. The present invention is directed to such a process, the materials and compositions required for implementation of the process, and the methods for production of the materials.